1 (a). Field of the Invention
The present invention relates to a graded refractive index lens having refractive index distribution in the direction along the optical axis.
2. (b). Description of the Prior Art
In the recent years, it is conceived to use graded refractive index lenses in various types of lens systems.
The graded refractive index lenses are classified roughly into the radial GRIN lens having refractive index distribution in the radial direction perpendicular to the optical axis and the axial GRIN lens having refractive index distribution in the direction along the optical axis. Out of these GRIN lenses, the axial GRIN lens can have a function similar to that of an aspherical surface simply by forming a spherical surface on the axial GRIN lens. Japanese Unexamined Published Patent Application No. 176905/61 discloses an axial GRIN lens which is used for correcting curvature of field. However, it is generally considered that the aberration correcting capability of the axial GRIN lens is lower than that of the radial GRIN lens. Especially, it is considered that the axial GRIN lens has no capability to correct chromatic aberration. See Applied Optics Vol. 21, No. 6, pages 993 to 998.
Out of optical systems, the simplest system consists of a single lens component. It is impossible to correct chromatic aberration with a single homogeneous lens component and, in order to correct chromatic aberration with an optical system consisting only of homogeneous lens components, it is necessary to combine at least two lens components.
FIG. 1 shows an example of the conventional imaging lens system consisting only of a single homogeneous lens component. This conventional lens system is designed with the numerical data listed below:
______________________________________ r.sup.1 = 5.5692 d.sub.1 = 1.8861 n.sub.1 = 1.49216 .nu..sub.1 = 57.5 r.sup.2 = 7.3108 (aspherical surface) d.sub.2 = 2.7943 r.sup.3 = .infin. (stop) aspherical surface coefficients P = 1.0945, B = 0, E = -0.33201 .times. 10.sup.-3 F = 0.52385 .times. 10.sup.-4, G = -0.26878 .times. 10.sup.-5 H = -0.11399 .times. 10.sup.-6, I = 0.67593 .times. 10.sup.-8 J = 0.12698 .times. 10.sup.-8, K = 0.25080 .times. 10.sup.-19 ______________________________________
The aspherical surface (the second surface) included in the above numerical data is expressed by the following formula: ##EQU1## wherein the x axis is taken, the optical axis, the s axis is located on a plane perpendicular to the optical axis, the origin is taken as the intersection between the x axis and the aspherical surface, the reference symbols B, E, F, G, H, I, J and K denote the aspherical surface coefficients, the reference symbol p represents the conic constant and the reference symbol e designates eccentricity. Aberration characteristics of the imaging lens system having the numerical data listed above are illustrated in FIG. 2, wherein aberrations, especially chromatic aberration, are remarkable and the imaging lens system cannot form a favorable image with the white light. In order to correct the chromatic aberration, it is conceivable to use a GRIN lens in the imaging lens system. However, all the GRIN lenses conventionally used for correcting the chromatic aberration are radial GRIN lenses and there is known no axial GRIN lens which is used singly for correcting the chromatic aberration.